What Size Boxes

I will be making a 3 tiered wedding cake which I will be stacking at the venue with seperators in between each one. Each cake is 6", 8" and 10" with each cake drum being the same size as the cake. Do I put each cake in the same size box or do I have them in a slightly larger box to avoid contact with the deocrations on the side of the cake?

Well, normally a cake drum is larger than the cake. You'd put the drum onto at least a cardboard that is larger and the box would be the size of the larger cardboard. Discard the cardboard when you stack the cake - its just for transport.